One of the completion procedures for an oil well is forming perforations into the formation to obtain production from the formation. One technique is to place a string of perforating guns in the borehole adjacent the zone to be produced, detonating the perforating guns to perforate through pipe in the well and into the formation to obtain production. A popular procedure in use of perforating guns is to run them into the well on tubing, this being identified as tubing conveyed perforating (TCP). TCP guns may be detonated by dropping a sinker bar through the tubing which strikes a mechanism at the top of the string of guns. This detonates an axially aligned shaped charge, ignites the primer cord to thereby detonate the perforating shaped charges. There is always the possibility of misfiring from the impact system, and when this occurs, it then requires that the TCP gun assembly be retrieved. This is an extremely dangerous procedure. For instance, great damage can be done to the well should the TCP guns accidentally trigger at the wrong depth. Even more dangerous, it is possible that the shaped charges may detonate above the wellhead during retrieval, sustaining injuries to the operating personnel. For these reasons, it is extremely dangerous to be forced into retrieval of armed TCP guns after malfunction of the firing mechanism.
This is an advance over and includes subject matter of an earlier application, being a continuation-in-part of Ser. No. 737,958 filed on May 28, 1985. The present version sets forth a different pressure operated firing system which serves as a safety or backup system. This and the prior backup system can be triggered through an alternate procedure. Accordingly, it is installed at the lower end of every TCP gun assembly, not used during proper operation of the primary firing mechanism (normally triggered by a dropped sinker bar), and yet is available for emergency use at any time. For safety sake, a backup system is then provided. The backup system operates completely differently than does the sinker bar triggered firing system. Thus, a safe backup procedure, assuming failure of the primary firing system, can be initiated by providing pressure of a specified level through the tubing which supports the TCP guns. A pressure sequence (increase and decrease) is initiated, thereby arming the detonator mechanism and then firing the backup mechanism to set off the TCP guns.
One reference of interest is U.S. Pat. No. 4,509,604 which sets forth a pressure responsive system, in part functioning on pressure in the annulus. Another reference of interest in U.S. Pat. No. 4,484,632 describing an alternate TCP system. As will be described below, the present apparatus utilizes a tubing pressure differential sequence to trigger detonation of the backup system. With the foregoing problem and need in view, the present apparatus is thus briefly summarized as a backup detonation system for TCP guns, the system responsive to a pressure sequence conveyed through the tubing string connected to the TCP guns and is particularly adapted for use on failure of the primary firing system. Other advantages and features of the present apparatus and its method of operation will become more readily apparent upon consideration of the written specification and the drawings which are described below.